Its to thick for the heat bubbles to escape thus they build up and release all at once creating a miniature chili explosion. Dont be lazy, heat that shit up in a pot.
It aint chili if it has beans in it dude. But yea just put a paper towel over it. Or they do sell little covers for that.
This guys owes everyone a single internet. I never have trouble microwaving chili, though. But it's better if you heat it in a pot, which has been mentioned.
Water turns to steam inside the beans which then exploded like legume balloons. IIRC this is the same process that make popcorn go pop.
My cousin told me about a time he was trying to use up eggs that he had in the fridge, so he put eggs on EVERYTHING. Burgers, chili, spaghetti, enchiladas, everything. His findings: Everything tastes good with a fried egg on top/in it. lol